Power control for crystal growing

ABSTRACT

Apparatus and process for crystal growing by the Czochralski or similar processes in which the operator determines the amount of power to be furnished to the heating element for the crystal material, and in which the system automatically maintains said furnished power at said determined amount. In this way shortrange disturbances in the heating process are compensated for, while long-range changes are taken care of by manual or automatic changes, from time to time, in the power level to be furnished. As a result, crystal diameter control is markedly improved on a production basis.

United States Patent [72] Inventor Richard hl 3,284,172 11/1966 Binder 23/301 New York,N-Y. 3,321,299 5/1967 Binder 75/10 1968 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-E. L. Weise [45] Patented 1971 Attorney-James and Franklin [73] Assignee Semimetals, Inc.

Westbury, N.Y.

[54] POWER CONTROL FOR CRYSTAL GROWING 14 Claims, 1 Drawing Fig.

- ABSTRACT: Apparatus and process for crystal growing by [52] [LS-Cl l48/L6, the Czochralski or similar processes i which the operator 23/273, 23/301 determines the amount of power to be furnished to the heating [51] 1131. C1 B01] 17/18 element for the crystal material, and in which the system auto [50] Fleld 01 Search 148/1.6; i n maintains i furnished power at i determined 23/273; 13/24; 23/301 amount. In this way short-range disturbances in the heating process are compensated for, while long-range changes are [56] References Clted taken care of by manual or automatic changes, from time to UNITED STATES PATENTS time, in the power level to be furnished. As a result, crystal 3,275,419 9/1966 Spielmann 23/301 diameter control is markedly improved on a production basis.

POM/L9 SDI/RC6 MAG/v SAclURABLl AMP]. Fl 1e ES so CONTROLLER 20 5:1- LEVEL 46 I /QQQQQ it 22 ho e 332 WATTMETER rmwsoucm POWER CONTROL FOR CRYSTAL GROWING The present invention relates to crystal growing and particularly to an apparatus and process for controlling the diameter of a crystal rod grown by the Czochralski or similar process.

In the Czochralski process of crystal growing, a quantity of crystal material is melted in a crucible by an external heating element. The crystallized material is drawn from the melted mass by a rod which is raised as the process continues, thereby to form a solid crystalline rod, which is later sliced into individual crystal wafers. Since the wafers should all be of the same size if they are to have the same electrical characteristics and diameter fluctuations usually cause resistivity fluctuations, it is highly desirable that the crystal rod be of uniform diameter. The diameter of the crystal rod produced by this process is determined by many factors, all of which affect the two primary factors: the heat being dissipated from the interface between the solid and liquid crystal phases, and the speed at which the solid crystal material is pulled awayfrom the melt. The heat removed from the junction over a period of time determines the volume of the melt which solidifies, while the pulled speed of the crystal detennines the length, and hence also the diameter, of that solidified volume.

Several mechanical-pulling arrangements are known which provide a precisely controlled pulling rate of the solid from the liquid phase of the crystal, so that aspect of the crystal growing process does not present a problem. The precise control of the heat dissipation from the liquid-solid interface of the crystal presents serious difficulties in the known crystal-growing apparatuses, because this parameter is affected by a large number of variables, many of which are not directly controllable. Two of these are variation in the voltage applied to the heating element, and variation in the resistance of the heating element. Slight variations in the rate of heat dissipation at the interface may produce nonuniformity of the crystal rod diameter.

In the past this problem has been approached from a temperature point of view-attempts have been made to maintain the liquid-solid interface at a constant temperature. To that end a temperature-sensing element such as a thermocouple or thermopile is physically arranged in proximity to the crucible and thus near the molten crystal material. That temperaturesensing element in turn controls a servosystem which varies the power furnished to the heating element in order to maintain the sensed temperature at a constant value.

The temperature-sensing approach of the prior art has not been satisfactory. Because the sensing element is located at a point spaced from the interface, there is neither a precise nor a stable relation between the sensed temperature and the actual interface temperature. Significant temperature differentials between the sensing element and the interface may develop which are proportional to the heat-transfer qualities of the crystal material, the ambient temperature conditions, and the volume of the molten material in the crucible, to name a few factors. in fact, because the solid-liquid interface can and does travel vertically in relation to the surface of the melt,

due to effects of surface tension, there is no one place in the melt-susceptor system which provides a stable temperature relationship to the interface temperature.

It is the prime object of the present invention to provide an apparatus and method for crystal growing in which greater uniformity of crystal diameter is achieved.

It is a further object of the present invention to provide a crystal-growing method and apparatus in which improved precision and reliability of the control of the heat dissipated at the liquid-solid interface is achieved.

It is another object of the present invention to provide a crystal-growing method and apparatus in which an increased volume of crystal material can be grown without'sacrificing precision of operation and uniformity of the end product.

It is a further object of the present invention to 'provide a crystal-growing method and apparatus in which the power supplied to the heater can be precisely set at a desired level and automatically maintained at that level; the said level being variable as desired as the crystal-growing process continues.

It is yet another object of the present invention to provide a crystal-growing method and apparatus which can be set and controlled by an operator more readily, and with greater crystal diameter accuracy than has heretofore been possible, thereby to increase greatly both the productivity of a given operator and the quality of the product.

The broad concept of the present invention is to depart from the temperature-measuring approach of the prior art. Instead, a constant power concept is adopted. The amount of power supplied to the heater, and hence the heat supplied to the melt, is automatically maintained at a desired preset value. Since the heat being dissipated from the junction is the summation of all heat flows across the junction, further elTorts are made to maintain the system in thermal stability. The cooling water for the crystal puller apparatus is maintained at a constant temperature, and the gas flow through the system is maintained at a constant temperature and flow. Thus, the primary factors affecting the heat dissipated from the interface are maintained constant, and any changes in the interface thermal characteristics result from changes due to the level of the melt receding and/or the top of the crystal rising into cooler regions. It may be necessary, therefore, to vary from time to time, the proper power level, but these changes need notbe frequent, because all short term, fast-acting variations are eliminated. These power level changes may be done manually, or automatically programmed after several manual runs. Such short term variations as changes in supply voltage or changes in heater resistance will be automatically compensated for. It may be necessary from time to time, as the volume of the melt decreases or for other reasons, to reset the proper power level, but this may be automatically programmed after a few manual runs.

The method and apparatus of the present invention are particularly suitable for use when the heater is of the resistance type, and is thus here specifically described. A signal representative of the power supplied to the heater is derived by sensing both the voltage applied to the heating element and the current flowing therethrough, and then producing a signal proportional to the product of the voltage and current and cosine of the phase shift between voltage and current. The predetermined value with which the power signal is compared is manually variable so that the operator can control the power level.

To the accomplishment of the above, and to such other objects as may hereinafter appear, the present invention relates to a crystal-growing method and apparatus as described in this specification, taken together with the accompanying drawing in which the single FIGURE is a schematic diagram of the crystal-growing growing apparatus of this invention.

A polycrystalline material 10 such as silicon or germanium, is placed within a susceptor 12, such as a quartz or graphite crucible. The interior of the lower portion of susceptor 12 is maintained at an elevated temperature above the melting point of the crystalline material 10 by means of heat developed by an electric heating element 14, here shown as a resistance heater, arranged in intimate heat-transfer relation with the susceptor l2, and operatively connected to an external, conventional AC power source 16. The upper portion of the susceptor 12 is maintained at a temperature below the melting point of the crystalline material so that the crystal rod 10a is maintained in a solid state as it is drawn or pulled from the molten crystalline material 10b by conventional pulling apparatus (not shown).

In accordance with the present invention, the heat dissipation at the liquid-solid interface 10c of the crystalline material 10 is precisely controlled by achieving a precise control of all major factors affecting the heat flow, including the power furnished to the heating element 14, which in turn is representative of the heat provided to the crystal melt 10b. The voltage applied across the heating element 14 and the current flowing therethrough are sensed and a signal is derived proportional to the product of this voltage and current and phase angle, and thus to the power furnished to heating element 14. This power signal-is then compared to a preset level and upon a detection of a deviation of the power signal from this preset, level, an adjustment is made automatically to the amount of power supplied from power source 16 to the heating element 14, thus to bring the power developed thereat back to the preset level.

In the exemplary system here specifically disclosed, the power from power source 16 is applied to the input of a saturable core reactor 18, the output terminalsof which are connected to a secondary winding 20 of a voltage stepdown transformer T.- The primary winding 22 of transformer T is connected by leads 24 and 26 to the heating element 14 to supply the electric power thereto. A pair of leads 28 and 30 is respectively connected to leads 24 and 26, and a current transformer winding 32 is placed in inductive relationship with lead 24. Thus, the voltage applied to the heating element 14 is also present across leads 28 and 30, and a current signal proportional to that heating element current is developed in the leads 33 and 34 connected to winding 32. Leads 28, 30 and leads 33, 34 are respectively connected to the voltage and current input terminals of wattmeter transducer 35 which in turn produces a voltage signal at its output terminal leads 36 and 38 which is proportional to the product of the voltage and current signals fed thereinto, and the cosine of the phase angle between said voltage and current signals. Thus, the voltage signal appearing across leads 36 and 38 is directly proportional to the instantaneous power being furnished to the heating element 14.

That voltage signal is operatively connected through a precision voltage divider 40 comprised of resistors R1 and R2, to reduce the level of the power signal by a known ratio, if required, and an R-C filter 42 comprising resistor R3 and capacitor C which removes the ripple component from this signal, to the input ofa controller 44. A preset signal is applied at 46 to controller 44 at a level corresponding to the desired rate of heat dissipation at the crystal liquid-solid interface 100. The controller 44 compares the level of the input power signal, corresponding to the power furnished to heating ele men! 14, to that preset level. When the power signal deviates from the preset level, the controller output, at leads 48 and 50, is adjusted either upwardly or downwardly in response to this sensed deviation. That output is applied to a magnetic amplifier 52 and is amplified thereby. The control current output of the magnetic amplifier 52 is then applied to a control winding formed on the core of the saturable core reactor 18, and is effective to control the proportion of the power from the external power source 16 applied across transformer T, and thus the amount of power provided to the heating element 14. Hence, any variation of the heating element power from its desired level will be detected and almost instantaneously corrected so that the heating element power will be substantially continuously maintained at its desired level. As a result the rate of heat dissipation at the crystal interface 10a will also remaincontinually and precisely maintained at its desired level, thereby insuring the precise and uniform formation of the crystal rod.

In this fashion the primary factors, and in particular the most rapidly varying factors, affecting the heat dissipated from the interface [c are automatically controlled. The remaining factors tending to disturb the rate of heat dissipation at the interface c are slowand long-range, and can readily be taken care of by manual or automatic routine adjustments of the preset level applied at 46 to the controller 44. This may be accomplished in any one ofa number of ways, as by connecting a manually or automatically controlled variable voltage source or potentiometer to the controller 44 at 46.

As a result of the use of the approach of the present invention, the uniformity of diameter of the crystal rods 10a improved markedly. With conventional temperature control the crystal rods had diameters which varied generally one-sixteenth inch, with occasional very wide or very narrow places. With the present invention rod diameter was routinely maintained at one thirty-second inch, with no marked wide or narrow places.

An additional advantage of the present system is that it permits a greater proportion of the melt 10b to become solidified than had heretofore been feasible. As the volume of the melt 10b decreases, the temperature stability decreases. This is improved to a significant degree by the system of the present invention where manual adjustment of the power level is not only possible but desirable. Typically 75-150 more grams of material are solidified with the present system, representing a saving of $30$ l 50 per melt.

Moreover, the system became much easier for the operators to control, with the result that a given operator could oversee and control more operations at the same time. Hence productivity is greatly increased, and accuracy in diameter maintenance not only does not sufi'er, but actually markedly improves.

it will be understood that the embodiment of the present invention here specifically disclosed is but exemplary, and that many changes may be made therein, all within the scope of the present invention as defined in the following claims:

I claim:

1. A crystal-growing apparatus comprising, a receptacle for holding a crystalline material to be melted, an electrical heating element arranged in heat-transfer relation to said receptacle, means for applying energy from an external power source to said heating element, means for deriving a signal proportional to the power developed at said heating element, and means for sensing the departure of said power signal from a predetermined value and for varying the amount of energy applied to said heating element in a sense such as to return said power signal to said predetermined value.

2. The apparatus of claim 1, in which said heating element is a resistance-heating element.

3. The apparatus of claim 2, comprising means for varying said predetermined value. v

4. The apparatus of claim 3, in which said power-signah deriving means comprises means for sensing the voltage applied to said heating element and the current flowing through said heating element and deriving therefrom said power-proportional signal.

5. The apparatus of claim 1, comprising means for varying said predetermined value.

6. The apparatus of claim 5, in which said power-signalderiving means comprises means for sensing the voltage applied to said heating element and the current flowing through said heating element and deriving therefrom said power-proportional signal.

7.. The apparatus of claim 1, in which said power-signaldriving means comprises means for sensing the voltage applied to said heating element and the current flowing through said heating element and deriving therefrom said power-propon tional signal.

8. In a process for crystal growing in which a material is melted within a receptacle by means of an electrical heating element arranged in heat-transfer relation to said receptacle and electrical energy is applied from a power source to said heating element, the steps which comprise, deriving a signal proportional to the power developed at said heating element, sensing the departure of said power signal from a predetermined value, and varying the amount of energy applied to said heating element in a sense to return said power signal to said predetermined value.

9. The method of claim 8, in which said heating element is a resistance-heating element.

10. The method of claim 9, comprising the step of varying said predetermined value.

11. The method of claim 10, in which said signal-deriving step comprises the steps of sensing the voltage applied to said heating element, sensing the current flowing through said heating element, and deriving therefrom said power signal.

l2.'The method of claim 8, comprising the step of varying said predetermined value.

13. The method of claim 12, in which said signal-deriving step comprises the steps of sensing the voltage applied to said heating element, sensing the current flowing through said heating element, and deriving therefrom said power signal.

14. The method of claim 8, in which said signal-deriving 5 step comprises the steps of sensing the voltage applied to said heating element, sensing the current flowing through said heating element, and deriving therefrom said power signal.

i t i 

2. The apparatus of claim 1, in which said heating element is a resistance-heating element.
 3. The apparatus of claim 2, comprising means for varying said predetermined value.
 4. The aPparatus of claim 3, in which said power-signal-deriving means comprises means for sensing the voltage applied to said heating element and the current flowing through said heating element and deriving therefrom said power-proportional signal.
 5. The apparatus of claim 1, comprising means for varying said predetermined value.
 6. The apparatus of claim 5, in which said power-signal-deriving means comprises means for sensing the voltage applied to said heating element and the current flowing through said heating element and deriving therefrom said power-proportional signal. 7.. The apparatus of claim 1, in which said power-signal-driving means comprises means for sensing the voltage applied to said heating element and the current flowing through said heating element and deriving therefrom said power-proportional signal.
 8. In a process for crystal growing in which a material is melted within a receptacle by means of an electrical heating element arranged in heat-transfer relation to said receptacle and electrical energy is applied from a power source to said heating element, the steps which comprise, deriving a signal proportional to the power developed at said heating element, sensing the departure of said power signal from a predetermined value, and varying the amount of energy applied to said heating element in a sense to return said power signal to said predetermined value.
 9. The method of claim 8, in which said heating element is a resistance-heating element.
 10. The method of claim 9, comprising the step of varying said predetermined value.
 11. The method of claim 10, in which said signal-deriving step comprises the steps of sensing the voltage applied to said heating element, sensing the current flowing through said heating element, and deriving therefrom said power signal.
 12. The method of claim 8, comprising the step of varying said predetermined value.
 13. The method of claim 12, in which said signal-deriving step comprises the steps of sensing the voltage applied to said heating element, sensing the current flowing through said heating element, and deriving therefrom said power signal.
 14. The method of claim 8, in which said signal-deriving step comprises the steps of sensing the voltage applied to said heating element, sensing the current flowing through said heating element, and deriving therefrom said power signal. 